<!doctype html>
<meta charset=utf-8>
<title>Transitions start at the same time regardless of on-screen status</title>
<link rel="help" href="https://drafts.csswg.org/css-animations-2/">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1888748">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
.transition {
  width: 100px;
  height: 100px;
  background-color: blue;
  transition: translate 10000ms linear;
  will-change: translate;
  translate: 0;
}
.offscreen {
  background-color: purple;
  translate: -1000px;
}
</style>
<div class="transition"></div>
<div class="transition offscreen"></div>
<script>
'use strict';

promise_test(async t => {
  let [a, b] = document.querySelectorAll(".transition");

  // Ensure a paint happens to make sure a transition gets triggered (otherwise
  // getAnimations() would flush sync, and maybe not trigger a transition if
  // it's the first style of the elements).
  await new Promise(r => requestAnimationFrame(() => requestAnimationFrame(r)));

  // Trigger both transitions at the same time.
  a.style.translate = b.style.translate = "100px";

  let ta = a.getAnimations()[0];
  let tb = b.getAnimations()[0];

  await Promise.all([ta.ready, tb.ready]);
  assert_equals(ta.startTime, tb.startTime, "Both transitions should've started at the same time");
});
</script>
